It actually uses themes from a lot of religious cultures: Japanese myths, Hindu gods, Norse gods, etc.

As for Kuja being Satan...while he disobeyed his "father" (Garland), his "father" was even more evil than he was. Was Kuja really evil? Didn't he repent more or less at the end of the game? Perhaps he was driven mad by the task his creator had given him, and still being seen as a lesser being.

BUT NOW I'M I'M OPENING THINGS UP FOR RELIGIOUS DEBATE, AND THAT'S NOT WHAT THIS THREAD IS ABOUT.